I. Field of the Invention
The present invention relates generally to mass gas flow sensors and, more particularly, to a mass gas flow sensor having on board digital signal processing means.
II. Description of Prior Art
There are many previously known mass gas flow sensors which provide analog output signals proportional to the mass of gas flow through the sensor. One such type of mass gas flow sensor is known as a hot wire sensor. Such hot wire sensors are frequently used in the automotive industry.
In the previously known hot wire sensors, the hot wire sensor includes a housing having a through bore through which a gas flows. Both a hot wire and a cold wire are positioned within a bypass bore in the housing while an analog electronic circuit attempts to maintain the temperature differential between the hot and cold wire at a predetermined amount.
In practice, gas flow through the housing bore cools the hot wire. Consequently, in order to maintain the temperature differential between the hot and cold wire, the current flow through the hot wire is increased by the electronic circuit in an attempt to maintain a constant temperature differential between the hot and cold wire. The increased current flow through the hot wire in effect forms a signal proportional to the mass gas flow through the housing bore.
These previously known hot wire sensors, however, all suffer from a number of disadvantages. One disadvantage of the previously known hot wire sensors is that the signal output from the hot wire sensor comprises a non-linear analog signal. This non-linear analog signal must be subsequently linearized by the main computer controlling the fuel control system before the output from the hot wire sensor can be effectively used by the fuel control system.
A still further disadvantage of these previously known hot wire sensors is that the electronic circuit controlling the current flow, and thus the temperature, of the hot wire are analog in design and frequently included a multiple operational amplifiers, trimming resistors and the like. These previously known analog circuits often times require individual calibration in order to meet acceptable performance standard. Furthermore, due to the analog nature of these previously known hot wire sensors, the accuracy of the hot wire sensor diminished over time.
A still further disadvantage of these previously known hot wire sensors is that it was necessary to individual design the analog electronic circuit for different types of gases such as air, methane, CNG (compressed natural gas), propane and the like. Consequently, it was difficult to adapt a single hot wire sensor to different types of gaseous flow.
A still further known disadvantage of these previously known hot wire sensors is that it was necessary to provide the fuel management system with an input signal representative of the temperature of the ambient gas, i.e. the temperature of the gas flowing across the cold wire. Consequently, it was necessary to include a thermocouple or other temperature sensing means in the hot wire sensor and to provide the output from the thermocouple or temperature sensor to the analog circuit controlling the current flow through the hot wire.